Process for the production of oxalic acid

ABSTRACT

Oxalic acid is produced by absorbing propylene in strong nitric acid and then heating the solution while removing the oxides of nitrogen formed, preferably after introducing oxygen into the solution.

United States Patent [151 3, 92,

Charamel et al. [4 1 Sept. 19, 1972 [54] PROCESS FOR THE PRODUCTION OF [51] Int. Cl ..C07c 51/32 OXALIC ACID [5 8] Held of Search ..260/533 R 72] Inventors: Andre Charamel; Jacques Marius Duroux, both of Lyon; Serge Siquet, [56] Referenms Cited ghaponost h all of nq FOREIGN PATENTS OR APPLICATIONS aques Descrolx, eceased, 181$, France Daniel Gras, Legal 742,053 5/1943 Germany ..260/533 R Representauve Primary Examiner-Lorraine A. Weinberger [73] Assignee: Rhone-Poulenc S.A., Paris, France Assistant Examiner-Richard D. Kelly Dec. 9 Att0rneyCuShman, & .CuShman [21] Appl. No.: 96,646 [57] ABSTRACT Related US, Application D t Oxalic acid is produced by absorbing propylene in strong nitric acid and then heating the solution while [63] Commuanon of 624,973 March removing the oxides of nitrogen formed, preferably 1967' after introducing oxygen into the solution.

[52] 7 Claims, 2 Drawing Figures US. Cl. ..260/533 R This is a continuation of application Ser. No. 624,973, filed Mar. 14, 1967. I

This invention relates to the manufacture of oxalic acid by oxidation of propylene.

It has already been proposed to prepare oxalic acid by the oxidation of propylene. Thus German Pat. Specification No. 742,053 describes a process for the oxidation of propylene with nitric acid at a temperature of between 50 and 70 C. in the presence or absence of oxygen, at normal or elevated pressure. In this single stage process, it is necessary to have sulphuric acid present if good yields (80 percent based on propylene converted, and 76 percent based on propylene employed) are to be obtained. In the absence of sulphuric acid, the yields based on propylene employed do not exceed 51.5 percent in the presence of oxygen at normalpressure, or 64 percent in the presence of oxygen under pressure. Furthermore, they do not exceed 44 percent, if oxygen is absent.

To overcome the disadvantages of this process, it has been proposed US. Pat. No. 3,081,345) to oxidize propylene in two stages:

l. The propylene is oxidized in liquid nitrogen peroxide at between -30 C. and +21 C. to produce intermediate oxidation products.

2. The intermediate products of the first stage are then oxidizedat 60 to 120 C. with an oxidizing agent which may be a sulphuric acid/nitric acid, mixture, nitric acid by itself, or nitrogen peroxide (if the operation is effected under pressure). However the achievement of good yields still depends on the use of sulphuric acid in the second stage, the yields dropping from 93 percent to 40-percent when sulphuric acid is omitted.

In these two known processes, the need to workin the presence of sulphuric acid brings with it numerous technological problems e.g., in the recovery and concentration of the mdtherliquors of "the oxidation, and the considerable corrosion of the equipment. Furthermore, the use of nitrogen peroxide in the process described in the abovementioned US. Patent involves numerous disadvantages because of its' low boiling point an the fact that it is not readily available.

The present invention provides a process by which oxalic acid may be prepared in good yields by oxidizing propylene with nitric acid without using sulphuric acid. The new process comprises introducing propylene into nitric acid, and then heating the solution obtained with simultaneous removal of the nitrogen oxides formed. The propylene may be introduced into the nitric acid, at atmospheric pressure or at a higher pressure, under the conditions described below. It is preferable, to obtain better yields, to subject the solution formed by dissolving the propylene in the nitric acid to a stream of oxygen (or oxygen-containing gas) at atmospheric or higher pressure to ensure oxidation of the nitrous fumes and the removal of the various nitrogen oxides.

By the new process, oxalic acid is obtained in good yields, which is unexpected for a process in which sulphuric acid is not used and the reactionconditions are such as to lower the concentration of nitrous fumes (N N 0, and N 0 and nitrous acid in the reaction medium, because in the earlier processes the achievement of good yields was dependant on the use of sulphuric acid and pure nitrogen peroxide or nitrogen peroxide mixed with nitric acid.

The concentration of nitric acid in the first stage of the process must not be too low. The yield of oxalic acid rapidly diminishes as the concentration of the nitric acid used decreases below 50 percent. On the other hand, there is no advantage in using nitric acid solutions of a higher concentration than 75 percent. Thus, in practice a nitric acid concentration of between 50 and 75 percent, and preferably 60-75 percent is very suitable. The molar ratio of propylene to 100 percent nitric acid may vary within quite wide limits, for example from 0.01:1 to 0.5:1, and preferably from 0.025:1 to 0.111. It is possible to work at atmospheric or higher pressure, but in practice an absolute pressure of 20 bars is not exceeded during this stage, and it is generally sufficient to work at an'absolute pressure of 1 to 10 bars. The temperature of the reaction medium may be from 10 to 40 C. and is preferably from 25 to 35 C. The rate of introduction of propylene into the nitric acid depends on numerous factors such as the conditions of pressure and temperature and the equipment used. The rate has to be so chosen that under the reaction conditions the propylene is absorbed as completely as possible, and in such a way that the molar ratio propylene/100 percent nitric acid adheres to the values indicated above. During this stage the propylene is completely dissolved and fixed in the nitric acid, intermediate products being formed.

As stated above, this first stage is preferably followed by a treatment of the reaction mixture with oxygen (or a gaseous mixture containing oxygen, for example air), carried out under the same temperature and pressure conditions as the first stage. The-amount of oxygen (or oxygen-containing gaseous mixture) used is preferably chosen so as to ensure both maximum oxidation of the nitrous fumes and nitrous acid produced during the first stage and as extensive as possible a. removal by entrainmentof the-nitrogen oxides. The purpose of this stage is to lower the concentration of N0 N 0 ,N O and HNO which-lead to degradation reactions, to the maximum extent and simultaneously to raise the nitric. acid concentration of the reaction mixture.

. fumes as completely as possible. This can be achieved by natural evolution when working at atmospheric pressure, or better still by displacement by bubbling a gas through the mixture as stated above, working under pressure with the introduction of a sweeping gas and a degassing process, or by working under pressure without the degassing but in the presence of oxygen to reoxidize the nitrous fumes in situ.

The oxalic acid resulting from this combination of operations is isolated by the usual techniques. For ex.- ample, the oxalic acid may be crystallized by cooling and the resulting crystals removed by filtration. After the water formed by the reaction has been removed, the mother liquors may be recycled to the first stage of a subsequent operation, at the same time as the nitric acid resulting from the reoxidation of the nitrous fumes formed during the process.

The reagents and the working conditions are particularly suitable for carrying out the process continuously. For example, the process may be operated as follows: nitric acid of a suitable concentration, and propylene, and continuously introduced into a first reactor so that the molar ratio of propyleriezl-INO remains within the values given above. After a dwell time depending on the reaction conditions, the reaction mixture is passed continuously to a second reactor into which a current of an oxygen-containing gas is passed. The degassed mixture is then passed continuously into a third apparatus where it is heated, with or without the simultaneous introduction of oxygen or air, at atmospheric pressure or optionally under pressure. The uncon verted nitrogen oxides driven out of the reaction mixture in the second and third stages are passed to an installation for the recovery of nitrous fumes, which ensures their reoxidation to nitric acid which is recycled to the firststage. The irrecoverable nitrogen (N and N is removed at this stage. The oxalic acid is recovered, and the mother liquors are recycled to the first stage after removing the water produced by the reaction by distillation. The introduction of fresh nitric acid is thus limited to replacing that which leaves the apparatus as irrecoverable nitrogen. However, and this is a further advantage of the new process, this cons'umption is low. It is from 0.8 kg. to 1.5 kg. of l-lNO per kilogram of oxalic acid dihydrate produced when working in three stages, and a little higher if the hot treatment is applied directly to the mixture obtained after absorption of the propylene in the nitric acid.

The following Examples, in which the gas flow rates are-flow rates expressed under normal conditions of temperature and pressure, illustrate the invention.

EXAMPLE 1 The equipment used is shown diagrammatically in FIG. 1 of the accompanying drawings. It consists of the following components: a first reactor comprising a cylindrical glass chamber 1 450 mm. high, of 30 mm. diameter and- 300 cm useful volume; a conical chamber 2 joined to the lower part of the cylindrical chamber and closed off where it joins the latter by a No. 3 sintered glass plate 3 (porosity l-40 .1.); a propylene supply tube 4 ending at the apex of the conical chamber and a nitric acid supply tube 5 ending above the plate 3; a double jacket 6 through which a liquid maintained at a constant temperature may be circulated; and a tube 7 for removing the reaction mixture by overflow, and a thermometer 8. A second reactor 9, identical to the first, is connected by a tube 7 with the first reactor, this tube arranged entering just above the sintered glass plate of the second reactor. The latter is also provided with an oxygen inlet 10, a tube 11 for removing the reaction mixture by overflow, and a tube 12 for removing the mixture of oxygen and nitrogen oxides evolved from the reaction mixture and passing them to a nitrous fumes recovery device. A conical two-liter flask 13 is closed by a ground stopper through which passes the tube 11 from the second reactor, a thermometer l4 and a tube 15 for removing the gas evolved by the reaction.

Before starting the process, water maintained at 20 C. is circulated through the double jacket 6, and the reactor 1 is then supplied with percent strength nitric acid at a rate of 284 g./hr. and with percent propylene at a rate of 4.68 l./hr. The level of the reaction mixture in the reactor 1 rises to the level of the overflow 7. The reaction mixture (gas liquid) then passes into the second reactor 9' into which a stream of oxygen is simultaneously introduced at a flow rate of 7.64 L/hr. through inlet 10. When the reaction mixture reaches the overflow 11, the apparatus is operational. Water at 20 C. is circulated in the jacket of the second reactor 9. The temperature of the reaction mixture in both reactors is, maintained at 25 C. The mixture which flows out of the second reactor 9 is collected in the conical flask 13 for 5 hours. During the whole of this period the mixture collected in 13 is stirred with a magnetic stirrer and kept at 55 C. In this way l,000 cm of mixture, reaching a depth of 7 cm. in flask 13, are collected. The supply to the reactors is then stopped, and heating of flask 13 is continued for 6 hours whilst the temperature is raised to 65 C. The nitrous fumes evolved are reoxidized to nitric acid in another apparatus. When this treatment is ended, the liquid mixture in the flask 13 is cooled to 20 C. and the oxalic acid in a sample is determined by the usual assay techniques, after the oxalic acid has been isolated by precipitation as calcium oxalate.

The reaction balance is as follows:

Nitric acid concentration at the end of the reaction in the product in the flask (13) (expressed as (HNO /HNO;,+H O) 55% Degree of conversion of propylene 100% Yield of oxalic acid based on propylene employed 71.4% Consumption of nitric acid. expressed as kg. of 100% HNO per kg. of oxalic acid dihydrate produced 1.13 kg/kg.

EXAMPLE 2 The procedure of Example 1 is followed but using nitric acid of 65 or 60 percent strength. The results are as follows:

The procedure of Example 1 is followed, the reaction 1 being fed with nitric acid of different concentrations, and the product issuing from the reactor 1 being directly collected in the flask 13, the second reactor 9 being disconnected. The mixture in flask 13 is worked up as in Example l. In this way the results given in the following Table are obtained:

Strength of Nitric acid consumption as Yield f li nitric acid kg. of 100% l-lNO per acid based on k kg. of oxalic acid dihydrate. propylene employed Though in this case the'consumption of'nitric acid is a little higher and the yield a little less good, the results obtained are still satisfactory.

EXAMPLE 4 The equipment used is -illustrated in FIG. 2. It comprises: a reactor 101 of useful volume 1,400 cm, consisting of a vertical cylindrical stainless steel chamber 102 of 40 mm. diameter and 550 mm. height, topped by a gas/liquid separation chamber 103, and joined at the top and bottom to a temperature control device consisting of a 12 mm. diameter tube 104 having a double jacket, the lower connecting conduit passing through a circulating pump 105. The reactor also comprises: a tube 106 for introducing nitric acid, a tube 107 for introducing propylene, a nozzle 108 passing vertically through the lower part of the reactor and connected to the tube 107 and the pump 105, and a tube 109, starting at the top of the chamber 103, for transferring the mixture into the next reactor. The latter consists of a stainless steel column 110, of 30 mm. diameter and 800 mm. height, packed with Raschig rings, and fitted with a lower side tube 111 connected to the tube 109 of the preceding reactor, a lower tube 112 for introducing air, a temperature control device made up, like that of the preceding reactor, of a tube with a double jacket 113 having a circulating pump 114 which can provide a flow rate of 50 l./hour, and a degassing tube 117 with a stop-cock 118 connected to an installation (not shown) for the recovery of the nitrous fumes. A tube 115 with a stop-cock 116 connected into the lower part of the temperature control tube permits continuous withdrawal of the mixture circulating in the assembly 110, 113. The withdrawn mixture is fed to a 2.5-liter stainless steel reactor (not shown) fitted with a stirrer turning at 280 rpm, a liquid inlet connected to the tube 115, a device for withdrawing the reaction product, and a tube for removing the nitrous fumes to a recovery device.

68.5 percent Strength nitric acid is introduced into the reactor 101 through the tube 106 at the rate of 1,100 g./hour, and propylene is introduced into the reactor through the tube 107 at the rate of 13.7 l./hour (i.e., 0.615 mol/hour). After a certain time the reactor is sufficiently full for it to be possible to circulate the mixture through the temperature control tube 104 and to maintain the temperature of the mixture at 25 C. When the mixture reaches the level of the removal tube 109, the mixture passes into the second reactor where air is injected at the rate of 100 l./hour through inlet 112. The temperature is maintained at 20 C. by the system 113. The pressure in 101 and 110 is maintained at bars absolute by adjusting the valve 1 18. When the equipment is operational, the reaction mixture is progressively withdrawn through tube 115 via stopcock 116, and collected in the 2.5-liter stainless steel reactor. A 2,246 g. sample is taken in this way, and kept at 55 C. for 5 hours and then at 65 C. for 5 hours. The product is cooled to C. The reaction balance is as follows:

Degree of conversion of propylene 100% Yield of oxalic acid based on propylene employed 70.2%

EXAMPLE 5 Three experiments A, B and C are carried out, under the conditions specified below, in anapparatus comprising two reactors identical to the reactors l and 9 of FIG. 1 using, in place of the flask 13, a 2.5 l. stainless steel autoclave having a stirrer system turning at 280 rpm, a liquid inlet, a dip tube gas inlet and a gas outlet with a valve for controlling the pressure.

The first reactor is supplied with percent strength nitric acid at the rate of g./hr. and with propylene in the proportion of 0.86 mol per kg. of the 75 percent strength nitric acid. The liquid issuing from the second reactor is first collected in a reservoir and then, when a sufficient reserve has been built up, it is passed at the rate of 800 g./hour into the autoclave which has beforehand been charged with 500 g. of 50 percent strength nitric acid. The contents of the autoclave are stirred and kept at 60 C. by circulating hot water through the double jacket of the autoclave. The conditions of treatment of the product in the autoclave in the various experiments are as follows:

EXPERIMENT A The mixture coming from the reservoir is collected for 3 hours 30 minutes in the autoclave which is connected to a device, operating at atmospheric pressure, for the recovery of nitrous fumes, the degassing valve being wide open. The material collected in this way is then heated for 4 hours at 60 C. The resulting material is cooled and worked up as in the preceding Examples.

EXPERIMENT B The product from the second reactor is introduced, by means of a pump, into the autoclave whose pressure release valve is adjusted to maintain a pressure of 7 bars absolute. The other working conditions are as in Experiment A.

EXPERIMENT C The procedure of Experiment B is followed, but with bubbling oxygen through the charge in the autoclave at a flow rate of l./hr.

The reaction balance for these three experiments is as follows:

Yield of oxalic acid based on propylene employed experiment Consumption of nitric acid,

in kg. of 100% HNO per kg.

of oxalic acid dihydrate.

3. The process of claim 1 in which the nitric acid concentration is 60 to 75 percent by weight.

4. The process of claim 1 in which the propylene is dissolved in nitric acid at a temperature of 2535 C.

5. The process of claim 1 in which the solution of propylene in nitric acid is heated at a temperature of 50-65 C.

6. The process of claim 1 in which oxygen is in- UNiTED STATES PATENT orrrm; CER'EIFICATE 0F CORECETWN Patent No. 3,692,830 Dated September 19, 1972 Inventor(s) Andre Charamel. Jacques Marius Duroux,

Serge Siquet and Jacques Descroix It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Please list on the face of the Official Grant the following priority data:

"March 17, 1966 France 53,888

Signed and sealed this 10th day of April 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The process of claim 1 in which oxygen is introduced into the solution formed by dissolving propylene in nitric acid, before or during the heating of the said solution.
 3. The process of claim 1 in which the nitric acid concentration is 60 to 75 percent by weight.
 4. The process of claim 1 in which the propylene is dissolved in nitric acid at a temperature of 25*-35* C.
 5. The process of claim 1 in which the solution of propylene in nitric acid is heated at a temperature of 50*-65* C.
 6. The process of claim 1 in which oxygen is introduced into the reaction mixture both at 10* to 40* C. after the propylene has been dissolved in the nitric acid, and at 45* to 100* C. while the solution obtained is being heated.
 7. The process of claim 1 in which the molar ratio is 0.025:1 to 0.1:1. 